Weighted Absorption Coefficient (αw)

The weighted absorption coefficient (αw) is a single-number rating defined in ISO 11654:1997, derived by fitting the measured absorption curve to a reference curve and reporting the value of the reference curve at 500 Hz. Shape indicators (L for low-frequency, M for mid-frequency, H for high-frequency) are appended when the measured curve exceeds the shifted reference curve by 0.25 or more in any frequency range. For example, αw = 0.85 (L) indicates strong overall absorption with notable excess absorption at low frequencies. This rating is widely used in European specifications and building codes. Unlike NRC, the reference curve method penalizes materials with uneven absorption spectra. αw also defines the sound absorption class (A through E) used in product labeling per ISO 11654.